CSP3: Consider adding a 'resource-src' directive

[jwatt]

SVG has the concept of resource documents, where an external document can contain things like markers, filters, gradients, etc., and other documents can link, say, the 'marker' CSS property to an anchor in the external document. UAs then load the external document, and draw the external marker as appropriate. Perhaps image-src should cover this case, but we should look at it more closely. At any rate, filing this while I remember.